Pleural mass forming extramedullary hematopoiesis masquerading as a malignant neoplasm

Extramedullary hematopoiesis (EMH) represents the presence of immature hematopoietic elements and their differentiation into mature blood components outside of the medullary bone and may be seen in a variety of circumstances in the postnatal period, but is most strongly associated with disorders of the hematopoietic system. Postnatally, EMH is typically identified at sites of fetal hematopoiesis, the spleen, and liver, but occasional reports have identified it in nearly every tissue of the body. We report a case of EMH presenting as pleural mass, initially suspected to represent a neoplastic process in a patient with multiple comorbidities, including history of carcinoma, but without co-existing hematologic disorder. On-site evaluation of the fine-needle aspiration specimen was initially suspicious for a malignant neoplasm, but further evaluation revealed the lesion to be a mass forming focus of non-hepatosplenic EMH. In the era of increasing utilization of imaging, mass forming EMH is increasingly detected. When unsuspected, EMH may present a diagnostic challenge for the pathologist and may be confused for a neoplastic process.